Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
FIG. 1 illustrates a typical vehicle powertrain system 10. Mechanical power flow connections are indicates with solid bold lines, the flow of hydraulic fluid is indicated with dashed lines, and the flow of electrical information signals is indicated with dotted lines. An internal combustion engine 12 drives a crankshaft 14 which supplies input power to transmission 16. The transmission 16 adjusts the speed and torque and delivers the power to differential 18. Differential 18 divides the power between left and rights wheels 20 and 22 while allowing slight speed differences as the vehicle turns a corner.
Within transmission 16, the speed and torque are adjusted by two components, torque converter 24 and gearbox 26. Torque converter 24 includes an impeller and turbine that transmit power hydro-dynamically whenever the impeller rotates faster than the turbine. It may also include a stator that multiplies the torque. The torque converter may also include a bypass clutch that, when engaged, transmits power mechanically from the impeller to the turbine without the parasitic losses associated with hydro-dynamic power transfer. Gearbox 26 includes gearing and clutches arranged such that engaging various subsets of the clutches establish various power flow paths. The different power flow paths have different speed ratios. Gearbox 26 shifts from one speed ratio to another speed ratio by releasing some clutches and engaging other clutches to establish a different power flow path.
Torque converter 24 and gearbox 26 are controlled by adjusting the pressure of hydraulic fluid supplied to various clutches. Pump 28 is driven by the transmission input which is driven by crankshaft 14. Pump 28 draws fluid from sump 30 and supplies the fluid, at elevated pressure, to valve body 32. The quantity of fluid supplies is based on engine speed and on a parameter of the pump geometry called pump displacement. In response to signals from controller 34, valve body 32 supplies the fluid to the various clutches in torque converter 24 and gearbox 26 at controlled pressures less than the pressure supplied by pump 28. The valve body also supplies fluid to the hydro-dynamic chamber of torque converter 24 and supplies fluid for lubrication to gearbox 26. Fluid travels from gearbox 26 and valve body 32 back to the sump 30 to complete the cycle. The quantity of fluid needed varies depending on the current operating state of the transmission. In response to these changes and in response to changes in engine speed, controller 34 may also direct valve body 32 to adjust the pump displacement.